April 2010 Lithosphere Highlights

Boulder, CO, USA – LITHOSPHERE carries on from earlier work suggesting a Late Cretaceous-Paleogene “Nevadaplano” similar to South America’s Altiplano; assesses the state of stress in Earth’s crust in North America; offers clues to understanding ancient collisions by mapping the depth domains of the Eastern Ghats Belt, India; presents a tectonic model for the northern Owyhee Mountains of Idaho; and confirms the dynamic nature of the middle crust of Colorado’s Wet Mountains and the 1.4-billion year-old-intrusive granites therein.

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The N-S-trending Andes surmount South American lithosphere above the east-descending Nazca plate. The highest mountains, underlain by ~70-km-thick crust, cap the Cordillera at 25 degrees S, an extremely arid region. In contrast, the precipitation-drenched fjordland at 45 degrees S is supported by ~35-km-thick crust. The Cascade and Sierra Nevada ranges in the western United States display comparable N-S trends and latitudinal rainfall patterns. Westerly winds supply abundant moisture to the northwest, but precipitation diminishes southward, producing increased aridity where the Sierra achieves its maximum regional elevation around Mount Whitney. Overthickened, now delaminating, ~42-55-km-thick crust of the southern Sierra exceeds that of the ~35-km-thick northern Sierra and the active Cascade arc. Contrasts in orogenic crustal thickness in California are not as marked as in the Andes because Sierran arc construction ceased near the end of Cretaceous time. Geologic, geochemical, and stable isotopic data suggest that a Nevadaplano occupied central plus southern Nevada plus western Utah in the rain shadow of the Late Cretaceous-Paleogene arc. The ~40-45-km-thick Colorado Plateau crust lies well downwind from the southern Sierra, and, depending on when it became elevated, it might represent part of a broader highland that collapsed during Neogene Basin and Range extension.

The nature and generation processes of earthquakes in stable continental regions, such as eastern North America, remain a major scientific challenge. As a result, the associated seismic hazard is also poorly understood. Mazzotti and Townend present a comparative analysis of the state of stress in Earth’s crust in ten major seismic zones of central and eastern North America. The comparison shows a systematic pattern: in half of the zones, the earthquake-generating stress and the borehole regional stress are in good agreement; in contrast, the other half of the zones show a systematic 30-50 degrees clockwise rotation of the earthquake-generating stress relative to the borehole regional stress. On the basis of stress magnitude measurements and calculations, Mazzotti and Townend suggest that this pattern may be due to the interaction of stresses generated by postglacial rebound from the last Ice Age with local stress concentrators, such as weak faults, in some specific seismic zones (e.g., Charlevoix, Canada, or central Virginia, USA). This study, the first quantitative comparison of earthquake-generating stress and borehole stress in eastern North America, provides tantalizing indications that particular seismic zones may be the locus of enduring stress and strain concentrations.

Remnant signatures related to paleo mountain building processes have great significance to geoscientists. Besides answering whether or not the plate tectonic forces in their present-day form have operated similarly in the geologic past, they offer clues to understanding ancient collisions. Utilizing a new variant of the S receiver function technique developed by the authors, Ramesh et al. map the depth domains of the Eastern Ghats Belt located close to the east coast margin of India to demonstrate that this region is indeed an ancient analogue of the Himalayas. Their research reveals that the upper mantle in the study region preserved structures reminiscent of subduction during the Mesoproterozoic (ca. 1.6 Ga), surviving subsequent extreme processes such as the Gondwanaland breakup (ca. 140 Ma) and the rapid movement of the Indian tectonic plate during the Cretaceous (ca. 90-65 Ma). This remarkable resilience of the Indian land mass became possible due to the presence of a mechanically strong, buoyant, thick (more than 200 km) lithospheric keel beneath the Indian shield. Therefore, simplistic models, such as ones suggesting that the thin Indian lithosphere advanced, cannot be used to explain the rapid drift of the Indian tectonic plate.

The northern Owyhee Mountains granitoid rocks of southwestern Idaho are interpreted to be the southward continuation of the western Idaho shear zone (WISZ). Similar to a well-studied section of the WISZ by McCall, the northern Owyhee Mountains display steep foliation and lineation orientations, deformation of 98-90 Ma plutons, steep strontium isotopic gradients, and syntectonic tonalite intrusions. However, the Owyhee Mountains have three major differences from the WISZ by McCall. Benford et al. present a simple tectonic model to explain why the northern Owyhee Mountains (1) have a significantly less well-developed solid-state strain fabric foliations; (2) a trend of 020 degrees rather than 000 degrees; and (3) a wider transition zone in initial strontium ratios from 0.704 to 0.708.

Approximately 1.4 billion years ago, a major thermal event affected most of southern North America and involved the generation and emplacement of large volumes of granite throughout a vast region of the continent. The tectonic setting of this event has long been debated, with the main issue being the seeming incompatibility of widespread evidence for deformation and the particular geochemical characteristics of the granites. Jones et al. present a study focused on numerous 1.4-billion-year-old granites exposed across a tilted section of the middle crust in the Wet Mountains of southern Colorado. They document widespread and long-lived deformation in outcrops that represent some of the deepest levels of exposure in the Rocky Mountains. Their results lead to a better understanding of the interactions between magmatism and deformation in an actively deforming region and confirm the dynamic nature of the crust into which 1.4-billion year-old-granites intruded. Furthermore, they revisit the debate about the tectonic setting of these granites and suggest some new alternatives that may ultimately help to resolve the issue.